1. Field of the Invention
The present invention relates to a Raman amplified optical communication system utilizing bi-directional Raman amplification with Raman pump wavelengths and directions designed to suppress crosstalk caused by four-wave mixing (FWM) among the Raman pumps.
2. Description of the Related Art
State-of-the-art wavelength division multiplexed (WDM) optical fiber transmission systems employ distributed Raman amplification (DRA) in addition to discrete amplifiers in repeater units. DRA partly compensates fiber losses along the transmission fiber and thus allows increasing the distance between discrete amplifiers or repeater units. DRA is based on stimulated Raman scattering, an inelastic scattering process between photons and optical phonons in which optical power is transferred from shorter to longer wavelengths.
FIG. 1 shows a typical Raman gain profile (Raman gain spectrum). The maximum power transfer occurs between wavelengths separated by 13.3 THz (about 100 nm in the 1550 nm region). Two pumping arrangements can be distinguished, as shown in FIG. 2 and FIG. 3.
FIG. 2 shows counter-directional pumping, where the pump light 112 propagates in opposite direction to the signal light (signal waves) 111 in a transmission fiber 101. In this case, a discrete amplifier 103 and a pump unit 104 are provided in a repeater unit on one side (output side) of the transmission fiber 101 and coupled to the transmission fiber 101 through an optical coupler 102. The pump unit 104 comprises a plurality of pump lasers of different wavelengths.
FIG. 3 shows co-directional pumping, where the pump light 113 propagates in the same direction as the signal light 111. In this case, a pump unit 106 is provided in a repeater unit on the other side (input side) of the transmission fiber 101 and coupled to the transmission fiber 101 through an optical coupler 105.
In state-of-the art systems, counter-propagation is commonly used in order to avoid the risk of pump-signal crosstalk. Two sources of pump-signal crosstalk can be distinguished:    a) intrinsic relative intensity noise (RIN) of the pump lasers that is transferred to the signals and    b) intra-channel crosstalk, i.e. inter-symbol interference, and inter-channel crosstalk due to bit pattern dependent pump depletion (W. Jiang and P. Ye, “Crosstalk in Fiber Raman Amplification for WDM Systems”, Journal of Lightwave Technology, vol. 7, pp. 1407–1411, September 1989).
U.S. patent application 2001/0036004 A1 disclosed a method for the reduction of the first type of crosstalk in configurations with co-propagating pumps. The second type of crosstalk can be suppressed by choosing the co-propagating pump wavelength such that its group velocity differs from that of the signals. If the walk-off between signal and pump is sufficiently high, the effect of the bit pattern dependent pump depletion averages out.
Employing a plurality of pumps of different wavelengths with suitable power allows a flat gain over a wide signal wavelength region as required in broadband WDM transmission systems (Y. Emori and S. Namiki, “100 nm bandwidth flat gain Raman amplifiers pumped and gain-equalized by 12-wavelength-channel WDM high power laser diodes”, Optical Fiber Communication Conference 1999, Technical Digest PD19/1-PD19/3 Suppl.).
Since the Raman pumping efficiency is polarization sensitive it is necessary to depolarize the pump light in order to suppress polarization dependent gain. Depolarization can be achieved by multiplexing two waves with orthogonal polarization of the same or of slightly different frequencies fp1, fp2 given by fp1=fp−δfp, fp2=fp+δfp, where δfp is up to 0.35 THz, as shown in FIG. 4. Later on, the term “depolarized pump” will be used for such pairs of multiplexed waves with slightly different frequencies and orthogonal polarization. As frequency of a depolarized pump the center frequency fp is used.
Four-wave mixing (FWM) (Optical Fiber Telecommunications IIIA, Academic Press, Kaminov and Koch, chapter 8, pp. 212–225, 1997) is a nonlinear process induced by the Kerr effect in optical fibers. If three signals at frequencies f1, f2, and f3 co-propagate through a single mode fiber, light at a frequency f4=f1+f2−f3 will be generated as shown in FIG. 5. The FWM power depends on    a) the optical frequencies, the optical input powers and the polarization states (Kyo Inoue, “Polarization effect on four-wave mixing efficiency in a single-mode fiber”, IEEE Journal of Quantum Electronics, Vol. 28, No. 4, pp. 883–894, 1992) of the three mixing waves, and    b) the dispersion, the nonlinear and the loss characteristics of the fiber.
The case f1=f2 is referred to as “degenerate” FWM. The efficiency of FWM strongly depends on the phase matching between the four interacting waves, which can be expressed by the phase matching parameter
                                 Δβ          =                                    β              3                        +                          β              4                        -                          β              1                        -                          β              2                                                                    =                                                    π                ⁢                                                                  ⁢                                  c                  2                                                            f                0                4                                      ⁢                          S              ⁡                              [                                                      (                                                                  f                        1                                            -                                              f                        0                                                              )                                    +                                      (                                                                  f                        2                                            -                                              f                        0                                                              )                                                  ]                                      ⁢                          (                                                f                  1                                -                                  f                  3                                            )                        ⁢                          (                                                f                  2                                -                                  f                  3                                            )                                          where βj (j=1, 2, 3, 4) are the propagation constants of the four waves, and c the light velocity and f0 the zero-dispersion frequency and S the dispersion slope (S=dDc/dλ) of the fiber. The FWM efficiency is highest in the case of phase matching (Δβ=0) which occurs if f1 and f2 are symmetrically allocated around f0.
FWM has been mainly recognized as a limiting factor among signal channels. However, FWM among pump lights can also affect the transmission performance in fibers with the zero dispersion wavelength in the pump wavelength region, as has been pointed out by Neuhauser et al. (R. E. Neuhauser, P. M. Krummrich, H. Bock, and C. Glingener, “Impact of nonlinear pump interactions on broadband distributed Raman amplification”, Optical Fiber Communication Conference 2001, Technical Digest MA4/1–MA4/3). Depending on the wavelength allocation of the Raman pump lights and the signal wavelengths, the FWM products of the Raman pumps can fall into the signal wavelength range, where they undergo Raman amplification. Thus, in the case of co-propagating pumping as shown in FIG. 3, these FWM products cause crosstalk with the signals.
FIG. 6 shows spectra in the co-propagating pumping scheme where pump light of frequencies f1, f2, f3, and f4 co-propagates in the same direction as L-band and C-band signal waves (forward direction). PIF, POB, and POF respectively represent a spectrum of forward propagating light at fiber input, a spectrum of backward propagating light at fiber output, and a spectrum of forward propagating light at fiber output. As illustrated, FWM products 121 appear at fiber output and cause crosstalk with the signal waves 122.
In the counter-propagating pumping scheme, the FWM products of the Raman pumps do not directly crosstalk with the signals. However, due to Rayleigh backscattering, a small part of the light is reflected into the opposite direction as shown in FIG. 7. When pump light 131 couter-propagating in opposite direction to signals 132 generates FWM products 133, light of the products is partially Rayleigh backscattered and generates backscattered FWM products 134, which again undergo Raman amplification and give rise to crosstalk with the signals 132.
FIG. 8 shows spectra in the counter-propagating pumping scheme shown in FIG. 2, where pump light of frequencies f1, f2, f3, and f4 counter-propagates in opposite direction to the L-band and C-band signal waves (backward direction). In this case, pump light appears in POB and generates Rayleigh backscattered FWM products 123 in POF, which cause crosstalk with the signal waves 122.
Thus, a method is required to suppress FWM among Raman pumps in fibers with the zero dispersion wavelength in the pump wavelength region.